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AIM

10/12/17

7

−2−1

Altimeter Setting Procedures

Section 2. Altimeter Setting Procedures

7

−2−1. General

a.

The accuracy of aircraft altimeters is subject to

the following factors:

1.

Nonstandard temperatures of the atmosphere.

2.

Nonstandard atmospheric pressure.

3.

Aircraft static pressure systems (position

error); and

4.

Instrument error.

b.

EXTREME CAUTION SHOULD BE EXER-

CISED WHEN FLYING IN PROXIMITY TO
OBSTRUCTIONS OR TERRAIN IN LOW TEM-
PERATURES AND PRESSURES. This is especially
true in extremely cold temperatures that cause a large
differential between the Standard Day temperature
and actual temperature. This circumstance can cause
serious errors that result in the aircraft being
significantly lower than the indicated altitude.

NOTE

Standard temperature at sea level is 15 degrees Celsius
(59 degrees Fahrenheit). The temperature gradient from
sea level is minus 2 degrees Celsius (3.6 degrees
Fahrenheit) per 1,000 feet. Pilots should apply corrections
for static pressure systems and/or instruments, if
appreciable errors exist.

c.

The adoption of a standard altimeter setting at

the higher altitudes eliminates station barometer
errors, some altimeter instrument errors, and errors
caused by altimeter settings derived from different
geographical sources.

7

−2−2. Procedures

The cruising altitude or flight level of aircraft must be
maintained by reference to an altimeter which must
be set, when operating:

a. Below 18,000 feet MSL.

1. When the barometric pressure is

31.00 inches Hg. or less.

To the current reported

altimeter setting of a station along the route and
within 100 NM of the aircraft, or if there is no station
within this area, the current reported altimeter setting
of an appropriate available station. When an aircraft

is en route on an instrument flight plan, air traffic
controllers will furnish this information to the pilot at
least once while the aircraft is in the controllers area
of jurisdiction. In the case of an aircraft not equipped
with a radio, set to the elevation of the departure
airport or use an appropriate altimeter setting
available prior to departure.

2. When the barometric pressure exceeds

31.00 inches Hg.

The following procedures will be

placed in effect by NOTAM defining the geographic
area affected:

(a) For all aircraft.

Set 31.00 inches for en

route operations below 18,000 feet MSL. Maintain
this setting until beyond the affected area or until
reaching final approach segment. At the beginning of
the final approach segment, the current altimeter
setting will be set, if possible. If not possible,
31.00 inches will remain set throughout the ap-
proach. Aircraft on departure or missed approach will
set 31.00 inches prior to reaching any mandatory/
crossing altitude or 1,500 feet AGL, whichever is
lower. (Air traffic control will issue actual altimeter
settings and advise pilots to set 31.00 inches in their
altimeters for en route operations below 18,000 feet
MSL in affected areas.)

(b)

During preflight, barometric altimeters

must be checked for normal operation to the extent
possible.

(c)

For aircraft with the capability of setting

the current altimeter setting and operating into
airports with the capability of measuring the current
altimeter setting, no additional restrictions apply.

(d)

For aircraft operating VFR, there are no

additional restrictions, however, extra diligence in
flight planning and in operating in these conditions is
essential.

(e)

Airports unable to accurately measure

barometric pressures above 31.00 inches of Hg. will
report the barometric pressure as “missing” or “in
excess of 31.00 inches of Hg.” Flight operations to
and from those airports are restricted to VFR weather
conditions.